1. Field of the Invention
The present invention generally relates to a thermo-sensitive recording apparatus for performing thermal recording by using a thermal head, and particularly to a thermo-sensitive recording apparatus in which thermal energy for printing can be corrected.
2. Description of the Prior Art
A recording apparatus for performing thermal recording by using a thermo-sensitive paper or transfer type thermo-sensitive recording medium is widely used for facsimile equipment, printers, etc. Generally, in such a recording apparatus, a thermal head in which a plurality of heater units or elements are arranged in one line is used as a recording head. Since the thermal head produces thermal energy for printing, there arises a problem of deterioration in picture quantity due to the energy. The causes for deterioration in picture quality may be classified into six factors as follows:
(1) Heat storage in thermal head; PA1 (2) Data of thermal history; PA1 (3) Temperature of substrate of thermal head; PA1 (4) Variations in resistance of heater elements; PA1 (5) Variations in recording interval; and PA1 (6) Voltage drop due to black percentage.
Heat storage in the thermal head means that at any one time the respective heater elements may be different from each other in heat storage depending on the pattern to be printed. The heat storage state of a heater element may be affected by other heater elements arranged close by.
The data of thermal history means the state of the head as a result of printing information on the preceding line. In a thermo-sensitive recording apparatus the pulse width or amplitude of a voltage pulse (printing pulse) controls operation of the thermal head. The thermal history of the head affects recording in the next line.
Temperature of the substrate of a thermal head means the temperature of a substrate of the thermal head on which a number of heater elements are formed.
Variations in resistance of heater elements mean variations in resistance resulting from manufacturing. There are two kinds of variations. One is variation in resistance among the heater elements in one thermal head, and the other is variation in the mean value of resistance among a plurality of thermal heads. The former variation may be about .+-.25% and the latter variation may fall within a range of 200 to 300.OMEGA..
Variations in recording interval mean variations in time interval from the starting of printing on one line to the starting of printing on the next line.
Finally, voltage drop due to black percentage means that the value of the voltage drop of a power source in energizing the heater elements varies depending on the rate or percentage of black dots occupying one line. As the source voltage decreases, the density of an image is lowered correspondingly.
Conventionally, thermal energy correction has been performed separately for the respective factors. For example, in a rapid recording type thermo-sensitive recording apparatus having a printing cycle equal to or shorter than 10 m sec., the printing operation may be started before latent heat has been sufficiently purged and heat storage in each heater element then causes a serious problem. In such an apparatus, therefore, the state of heat storage was calculated to vary the pulse width or amplitude of the recording pulse to be applied to each heater element to control the thermal energy applied to the same. Alternatively, in a recording apparatus connected to a computer, the recording interval may largely vary for various data processing operations. In such an apparatus, for example, a slight current was caused to flow in the thermal head during non-printing periods to prevent a large variation in temperature of each heater element due to the lapse of time.
Thus, thermal energy correction has been effected separately for the above-mentioned factors in the conventional thermo-sensitive recording apparatus. There has been no effective countermeasures when a combination of the different factors have caused deterioration in picture quantity in one apparatus. A combination of various correction means applied separately to counteract the factors may attain satisfactory effects under certain circumstances. Nonetheless, there is a danger of deterioration in picture quantity due to an excess or deficiency of heat generation in each heater element if attention is paid to individual heater elements.